Water-absorbing resins are popularly used as water-absorbing agent absorbing body fluids (urine, blood, etc.) and the like, providing major components for paper diapers, sanitary napkins, incontinence pads, and other hygiene materials.
Recent years have seen development of deodorizing or otherwise value-added water-absorbing resins in response to increasing demands for adult disposable diapers from aging populations. Also under development are diapers which feel comfortable when worn. These diapers well fit body shape and allow no leak. These enhancements are mostly due to structural improvements of the absorbent, but partly attributable to improved performance of the water-absorbing resin.
The performance includes, for example, seepage and diffusing absorbency. Related improvements have been made in terms of centrifuge retention capacity (CRC), absorbency against pressure (AAP), gel strength, and absorption rate.
Document 1, or U.S. Pat. No. 5,147,343, discloses a porous fiber matrix and an absorbent composition made from a superabsorbent polymer dispersed in the pores in the fiber matrix. The superabsorbent polymer can absorb at least 27 ml of a 0.9 wt. % solution of sodium chloride against a pressure of 21000 dyne/cm2. The absorbent composition advantageously contains a 10 wt. % to 60 wt. % superabsorbent polymer.
Document 2, or U.S. Pat. No. 5,149,335, describes an absorbent structure of a 60 wt. % to 100 wt. % superabsorbent polymer. The superabsorbent polymer is characterized by a free swell rate of less than 60 seconds and at least a 5-minutes AUL (absorption under load) of 15 g/g.
Document 3, or European patent 0532002, discloses a porous fiber matrix and an absorbent composition containing an at least 30 wt. % superabsorbent polymer. The superabsorbent polymer has a deformation under load of less than 0.60 mm and a wicking index of 10 cm or more.
Document 4, or European patent 0615736, relates to an absorbent composition containing a 30 wt. % to 100 wt. % superabsorbent polymer which has a pressure absorbency index of at least 100 and an extractable content of less than 13 wt. % (during a 16-hour extraction in a 0.9 wt. % aqueous solution of sodium chloride).
Document 5, or European patent 0761191, describes a fiber matrix and an absorbent composition containing an at least 30 wt. %, superabsorbent polymer which has a suction parameter of 700 or more.
Document 6, or U.S. Pat. No. 6,297,335, discloses an absorbent composition with a pressure absorbency index of less than 100 and a vertical absorption of at least 12 g/g against a pressure of 1922.8 Pa.
Document 7, or U.S. Pat. No. 5,599,335, discloses an absorbent composition with a saline flow conductivity (SFC) of at least about 30×10−7 cm3 sec/g for a 0.69 wt. % aqueous solution of sodium chloride and a performance of at least 23 g/g under a load of 0.7 psi (performance under load, or PUP).
One of obvious conditions of comfort in wearing a diaper or similar absorbent is quick absorption of urine or other body fluids after excretion. The foregoing, conventional methods focus on the water-absorption performance of the water-absorbing resin or absorbent such as the diaper. This approach alone however will fall short of providing sufficient comfort when the diaper or like absorbent is worn. Examining causes of discomfort, we have found that a “drop in temperature” of the absorbent after the diaper or like absorbent absorbs excreted urine or other body fluids, that is, the “cool feel” of the wearer, is the factor that seriously affects comfort. No conventional absorbents can prevent a drop in their temperature. Heat retention has not been considered. None of them have achieved a satisfactory level of heat retention in actual use.
Sacrificing absorption performance to give high heat retention would result in the water-absorbing resin failing to accomplish its basic goals of absorbing body fluids (urine, blood, etc.). Therefore, it is both important to give high heat retention and attain a required level of absorption performance.
As in the foregoing, a lot of improvements have been made on the absorption performance of the water-absorbing resin.
One of obvious conditions for a diaper or similar absorbent that is comfortable to wear is quick absorption of urine or other body fluids after excretion. However, many absorbents after being wet cool down (toward room temperature) with time after the absorption of a body fluid (generally at about body temperature). The cool feel is never comfortable. To put it differently, the wearer's problem, that is, discomfort, will be eased if the absorbent remains near the body temperature level for an extended period after the absorption of a body fluid.